UV-mapping. How I can get the correct scale on the x axis?
Alright, here you go, I assume this is what you're looking for?



To achieve what I did in the above picture, you have to mark the seams on the sides of the curve, like so:



Notice how the edges I selected turns red. The red indicates edges that are UV seams, meaning that when you unwrap your mesh to a UV image, it "cuts" along the red lines.

Then you simply unwrap it with the "Follow Active Quads" option, and there you go!


Hope this helps.